1. Field of the Invention
The invention relates to the reading of bar code patterns applied to carriers for the carriers to be automatically recognized. It concerns a method of detecting a bar code from a bar code signal which essentially forms a cross-section of a bar code pattern which, through irradiation, luminesces from the background of a carrier. The invention also comprises an apparatus for reading such a bar code pattern.
2. Description of the Prior Art
In automatic postal processing systems, as is well known, bar coding is used for sorting according to destination, for instance. To that end, at the input of such a system, for instance by means of video coding, each letter to be processed in such a system is provided with a processing code in bar code form. The processing code may be a destination code, as a postcode, derived from the destination address provided on the letter. At one or more decision points in the process the bar code is read. Reading the code basically comprises the following steps:
a. picking up an image signal of the physical bar pattern on the carrier by passing it along optical scanning means; PA1 b. detecting the bar pattern from the image signal and indicating, for instance in digital form, "bar/no bar" and, if applicable, the type of bar (e.g. thick/thin), for each position in the bar pattern; PA1 c. decoding the detected bar pattern.
On the one hand, a bar pattern provided on the carrier should be as inconspicuous as possible, but on the other it should be readily distinguishable from any other printing when read automatically. Accordingly, such bars are typically applied to a carrier in an ink that emits light under luminescent, particularly fluorescent, effect. A bar code signal of a luminescent bar code pattern can be read using transducing means such as known, for instance from Dutch patent specification NL 164980. For the bar pattern on the carrier to luminesce, it is to be subjected to focussed irradiation using UV light, for instance. Here, a specific problem arises, namely that of background influence due to such irradiation. This means that irradiation will not only cause the bars written in fluorescent ink to luminesce, but also their background, wholly or locally, which is a fact to be taken account of. This is the case when envelopes used for letters are made of paper containing so-called "whiteners", which have fluorescent properties. The same problem presents itself when other writing or printing in fluorescent ink extends into the zone of the letter where the bar pattern is applied. Moreover, it has turned out that a luminescent background may act as an amplifier of the luminescent effect of the bars themselves. Major variances may then arise in the signal amplitude of the image signal read, not only in bar code signals of successive letters, but even within one and the same bar code signal. This may weaken the reliability of the signal information used to make "bar/no bar" decisions.
When the bar code used is of the `mark space` type, the background influence also makes it more difficult to detect spaces in a bar pattern. In short, the problem is basically one of finding a reliable signal threshold or another criterion for each "bar/no bar" decision to be taken.